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The first part of this work evaluates Al-doped ZnO (AZO) as an optically transparent top-gate material for studies on semiconductor quantum dots. In comparison with conventional Ti gates, samples with AZO gates demonstrate a more than three times higher intensity in the quantum dot emission under comparable excitation conditions. On the other hand, charges inside a process-induced oxide layer at the interface to the semiconductor cause artifacts at gate voltages above  U ≈  1 V. The second part describes an optical and simulation study of a vertical electric-field (F)-induced switching from a strong to an asymmetric strong–weak confinement in GaAs cone-shell quantum dots (CSQDs), where the charge carrier probability densities are localized on the surface of a cone. These experiments are performed at low U and show no indications of an influence of interface charges. For a large F, the measured radiative lifetimes are substantially shorter compared with simulation results. We attribute this discrepancy to an F-induced transformation of the shape of the hole probability density. In detail, an increasing F pushes the hole into the wing part of a CSQD, where it forms a quantum ring. Accordingly, the confinement of the hole is changed from strong, which is assumed in the simulations, to weak, where the local radius is larger than the bulk exciton Bohr radius. In contrast to the hole, an increasing F pushes the electron into the CSQD tip, where it remains in a strong confinement. This means the radiative lifetime for large F is given by an asymmetric confinement with a strongly confined electron and a hole in a weak confinement. To our knowledge, this asymmetric strong–weak confinement represents a novel kind of quantum mechanical confinement and has not been observed so far. Furthermore, the observed weak confinement for the hole represents a confirmation of the theoretically predicted transformation of the hole probability density from a quantum dot into a quantum ring. For such quantum rings, application as storage for photo-excited charge carriers is predicted, which can be interesting for future quantum photonic integrated circuits. [ABSTRACT FROM AUTHOR]

Temperature field engineering provides a robust framework for investigating the behavior of phonons and excitons within two-dimensional (2D) layered van der Waals materials. Herein, we measured the photoluminescence (PL) spectroscopy of few-layer WSe2 and WSe2/graphene van der Waals heterostructures at various temperatures (from 10 to 300 K). The results indicate that the PL intensity of the heterostructure is lower than that of few-layer WSe2, which is attributed to charge transfer at the interface under photoexcitation conditions. The PL spectroscopy spectrum of the heterostructure is primarily composed of four distinct spectral features, arising from distinct phonon-assisted processes, direct recombination, and excitonic contributions. Furthermore, these physical processes can be modulated by varying the temperature. For instance, the phonon-assisted PL process gradually diminishes with the decrease in temperature, whereas the excitonic PL process emerges more distinctly with the reduction in temperature. These findings can provide support for understanding the photophysical processes in 2D heterostructure materials and lay the groundwork for optoelectronic devices based on nanomaterials. [ABSTRACT FROM AUTHOR]

Strain-free GaAs cone-shell quantum dots have a unique shape, which allows a wide tunability of the charge-carrier probability densities by external electric and magnetic fields. Here, the influence of a lateral electric field on the optical emission is studied experimentally using simulations. The simulations predict that the electron and hole form a lateral dipole when subjected to a lateral electric field. To evaluate this prediction experimentally, we integrate the dots in a lateral gate geometry and measure the Stark-shift of the exciton energy, the exciton intensity, the radiative lifetime, and the fine-structure splitting (FSS) using single-dot photoluminescence spectroscopy. The respective gate voltage dependencies show nontrivial trends with three pronounced regimes. We assume that the respective dominant processes are charge-carrier deformation at a low gate voltage U, a vertical charge-carrier shift at medium U, and a lateral charge-carrier polarization at high U. The lateral polarization forms a dipole, which can either enhance or compensate the intrinsic FSS induced by the QD shape anisotropy, dependent on the in-plane orientation of the electric field. Furthermore, the data show that the biexciton peak can be suppressed by a lateral gate voltage, and we assume the presence of an additional vertical electric field induced by surface charges. [ABSTRACT FROM AUTHOR]

Nanolayers of MoS2 can be grown to be used as active elements in nano-optoelectronic devices such as two-dimensional (2D) light emitters and optical detectors. The growth of 2D flakes might result in the formation of not only isolated triangles but also complex polycrystal flakes when different flakes interact during their in-plane expansion. In this paper, we investigate how monolayer MoS2 flakes of different shapes are affected by the biaxial strain resulting from the cooling process after chemical vapor deposition growth. The single- and polycrystal flakes are characterized at the nanoscale level by correlating morphological, electrical, and optical measurements and imaging. The main focus is given to the analysis of the exciton/trion photoluminescence (PL) components extracted from the spectra at different areas of the flake surface. According to the Raman imaging, the whole flake has built-in heterogeneous tensile strain, with the perimeter and the grain boundaries between the single-crystal parts of the poly flakes exhibiting a lower strain level (0.2–0.3%) and the central area being more strained (∼0.4%). At the perimeter and grain boundaries, the PL undergoes the strain-related blueshift accompanied by a weakening of the contribution of the long-wave trion to the spectrum and the trion binding energy. The trion formation is known to be proportional to a local electron concentration. The trion PL imaging compared to the surface potential mapping confirms a decrease in n-doping at the perimeter and grain boundaries, leading to the trion weakening. To confirm the results of electron drift to strained areas of the flake, creating the trion, the band bending at the tensile-strained flake has been theoretically calculated and modeled. The effect of edge defects at the perimeter and grain boundaries on the doping, which leads to the enhancement or inhibition of the trion formation depending on the edge and grain boundary interface type, is also discussed. [ABSTRACT FROM AUTHOR]

Plexcitonic systems based on metal nanostructures and molecular J-aggregates offer an excellent opportunity to explore the intriguing interplay between plasmonic excitations and excitons, offering unique insights into light–matter interactions at the nanoscale. Their potential applications in photocatalysis have prompted a growing interest in both their synthesis and the analysis of their properties. However, in order to construct a high-performing system, it is essential to ensure chemical and spectral compatibility between both components. We present the results of a study into a hybrid system, achieved through the coupling of gold nanobipyramids with organic molecules, and demonstrate the strengthened photochemical properties of such a system in comparison with purely J-aggregates. Our analysis includes the absorbance and photoluminescence characterization of the system, revealing the remarkable plexcitonic interaction and pronounced coupling effect. The absorbance spectroscopy of the hybrid systems enabled the investigation of the coupling strength (g). Additionally, the photoluminescence response of the J-aggregates and coupled systems reveals the impact of the coupling regime. Utilizing fluorescence lifetime imaging microscopy, we established how the photoluminescence lifetime components of the J-aggregates are affected within the plexcitonic system. Finally, to assess the photodegradation of J-aggregates and plexcitonic systems, we conducted a comparative analysis. Our findings reveal that plasmon-enhanced interactions lead to improved photostability in hybrid systems. [ABSTRACT FROM AUTHOR]

The temperature-dependent intensities of the exciton (X) and biexciton (XX) peaks from single GaAs cone–shell quantum dots (QDs) are studied with micro photoluminescence (PL) at varied excitation power and QD size. The QDs are fabricated by filling self-assembled nanoholes, which are drilled in an AlGaAs barrier by local droplet etching (LDE) during molecular beam epitaxy (MBE). This method allows the fabrication of strain-free QDs with sizes precisely controlled by the amount of material deposited for hole filling. Starting from the base temperature T = 3.2 K of the cryostat, single-dot PL measurements demonstrate a strong enhancement of the exciton emission up to a factor of five with increasing T. Both the maximum exciton intensity and the temperature T x , m a x of the maximum intensity depend on excitation power and dot size. At an elevated excitation power, T x , m a x becomes larger than 30 K. This allows an operation using an inexpensive and compact Stirling cryocooler. Above T x , m a x , the exciton intensity decreases strongly until it disappears. The experimental data are quantitatively reproduced by a model which considers the competing processes of exciton generation, annihilation, and recombination. Exciton generation in the QDs is achieved by the sum of direct excitation in the dot, plus additional bulk excitons diffusing from the barrier layers into the dot. The thermally driven bulk-exciton diffusion from the barriers causes the temperature enhancement of the exciton emission. Above T x , m a x , the intensity decreases due to exciton annihilation processes. In comparison to the exciton, the biexciton intensity shows only very weak enhancement, which is attributed to more efficient annihilation processes. [ABSTRACT FROM AUTHOR]

Achieving a better understanding of the physics of defects in halide perovskites (HPs) is a key challenge for improving the efficiency of the devices. Herein, a comprehensive study of the defect emission of CH3NH3PbI3 perovskite single crystals is presented. The emission of the pristine surface of cleaved crystals is systematically investigated based on steady‐state and time‐resolved micro‐photoluminescence (micro‐PL) spectroscopy. Donor–acceptor pair (DAP) recombination is observed due to the presence of native shallow defects. The DAP spectra present an important variability depending on the location on the surface of the crystals due to inhomogeneous defect distribution. A strong blueshift of the emission is measured as a function of excitation power and is explained by fluctuating potential caused by compensated defects. With increasing photocarrier density, a transition from a structureless to a structured DAP emission with several longitudinal optical phonon replicas is observed. The DAP transition is characterized by the redshift of the emission with time and a slow, non‐exponential PL decay. Sharp discrete lines with sub‐meV widths, the most recognizable spectral signature of DAP transition, are revealed. Herein, the apparently contradictory previous observations on defect emission of CH3NH3PbI3 are reconciled by the results and new insights are provided into the properties of defects in HPs. [ABSTRACT FROM AUTHOR]

The formation of structures of different sizes on the surface of a single sample and the study of the mechanisms of their excitations and energy transitions is a promising area of research. ZnO/SiO2/Si heterostructures were formed by two-stage electrochemical etching of silicon wafers and synthesizing zinc oxide nanoparticles using sol-gel technology. By scanning electron and atomic force microscopies, various pores and clusters were identified. The presence of five levels of surface hierarchy provided the synthesis of light-emitting particles with different properties. Light emission mechanisms are associated with the recombination of excitons and the hyperfine structure of charged particles trapped on oxygen vacancy. [ABSTRACT FROM AUTHOR]

The current work used the effective mass approximation conjoined with the finite element method to study the exciton states in a conical GaAs quantum dot. In particular, the dependence of the exciton energy on the geometrical parameters of a conical quantum dot has been studied. Once the one-particle eigenvalue equations have been solved, both for electrons and holes, the available information on energies and wave functions is used as input to calculate exciton energy and the effective band gap of the system. The lifetime of an exciton in a conical quantum dot has been estimated and shown to be in the range of nanoseconds. In addition, exciton-related Raman scattering, interband light absorption and photoluminescence in conical GaAs quantum dots have been calculated. It has been shown that with a decrease in the size of the quantum dot, the absorption peak has a blue shift, which is more pronounced for quantum dots of smaller sizes. Furthermore, the interband optical absorption and photoluminescence spectra have been revealed for different sizes of GaAs quantum dot. [ABSTRACT FROM AUTHOR]

A relation has been established between the decrement of the low-field magnetic spin effect (measured by the luminescence of polymer films of composites with rubrene microcrystals), caused by the addition of magnetic Cu–Ni nanoparticles with a low Curie temperature (TC = 40–60°C), and the photoinduced particle magnetic moment exceeding the dark moment. Based on the study of the temperature dependence of the decrement and its comparison with the thermal demagnetization of the dark magnetic moment, a conclusion was made about a possible mechanism for the photothermal magnetization of nanoparticles. [ABSTRACT FROM AUTHOR]

Layered materials have attracted significant interest because of their unique properties. Van der Waals heterostructures based on transition-metal dichalcogenides have been extensively studied because of potential optoelectronic applications. We investigate the optical response of a light-emitting tunneling structure based on a WSe 2 monolayer as an active emission material using the photoluminescence (PL) and electroluminescence (EL) experiments performed at low temperature of 5 K. We found that the application of the bias voltage allows us to change both a sign and a value of free carriers concentrations. Consequently, we address the several excitonic complexes emerging in PL spectra under applied bias voltage. The EL signal was also detected and ascribed to the emission in a high-carrier-concentration regime. The results show that the excitation mechanisms in the PL and EL are different, resulting in various emissions in both types of experimental techniques. • The optical response of a tunneling structure based on a WSe 2 monolayer was studied. • The emission lines apparent in the photoluminescence spectra were identified. • The electroluminescence in the high electrical bias regime was analyzed. [ABSTRACT FROM AUTHOR]

Semiconductor quantum dots of the A2B6 group and organic semiconductors have been widely studied and applied in optoelectronics. This study aims to combine CdTe quantum dots and perylene-based dye molecules into advanced nanostructure system targeting to improve their functional properties. In such systems, new electronic states, a mixture of Wannier–Mott excitons with charge-transfer excitons, have appeared at the interface of CdTe quantum dots and the perylene dye. The nature of such new states has been analyzed by absorption and photoluminescence spectroscopy with picosecond time resolution. Furthermore, aggregation of perylene dye on the CdTe has been elucidated, and contribution of Förster resonant energy transfer has been observed between aggregated forms of the dye and CdTe quantum dots in the hybrid CdTe-perylene nanostructures. The studied nanostructures have strongly quenched emission of quantum dots enabling potential application of such systems in dissociative sensing. [ABSTRACT FROM AUTHOR]

Strain-free GaAs quantum dots (QDs) are fabricated by filling droplet-etched nanoholes in AlGaAs. Using a template of nominally identical nanoholes, the QD size is precisely controlled by the thickness of the GaAs filling layer. Atomic force microscopy indicates that the QDs have a cone-shell shape. From single-dot photoluminescence measurements, values of the exciton emission energy (1.58...1.82 eV), the exciton–biexciton splitting (1.8...2.5 meV), the exciton radiative lifetime of bright (0.37...0.58 ns) and dark (3.2...6.7 ns) states, the quantum efficiency (0.89...0.92), and the oscillator strength (11.2...17.1) are determined as a function of the dot size. The experimental data are interpreted by comparison with an atomistic model. [ABSTRACT FROM AUTHOR]

Recent studies of transition-metal dichalcogenide (TMD) nanoribbons have stimulated the development of synthetic strategies for the controlled growth of these dimensionally restricted crystals. We demonstrate the width-controlled synthesis of MoSe2 nanoribbons grown on a designer surface comprising Si(001) treated with phosphine. Adjustment of the H2 partial pressure in the carrier gas stream enables the nanoribbon widths to be tuned between 175 nm and almost 500 nm. Experiments and simulations suggest that H2 exposure increases the surface coverage of hydrogen on the Si–P dimers that normally serve as favorable regions for nanoribbon nucleation and growth. Moreover, the MoSe2 nanoribbons exhibit an anomalous photoluminescence blue shift whose magnitude of 60 meV is similar to that reported in optical emission spectra of MoS2 nanoribbons. These studies demonstrate that the recently developed strategy of substrate-directed growth of nanoribbons can be extended to the selenide family of TMDs. Moreover, they expand the synthetic foundation for preparing complex TMD heterostructures, which are required for optical- and quantum-based sensors, transducers, and processors. [ABSTRACT FROM AUTHOR]

Heterointerlayer excitons (HIXs) have been intensively studied in heterostructures of various two-dimensional (2D) nanosystems with staggered band alignment (type II). In this study, the laser confocal microscopy (LCM) photoluminescence (PL) characteristics of exciton species, including HIXs in monolayer (1 L) WS2 and multilayer PbI2 heterostructures, were investigated from 3 to 293 K. The drastic decrease in the PL intensity of 1 L WS2 after hybridization with multilayer PbI2 indicates the occurrence of charge transfer. PL peaks corresponding to the neutral excitons (X0), trions (X–), and biexcitons (XX) of 1 L WS2 (i.e., intralayer excitons) were observed in the heterostructure at 3 K at 594, 601, and 606 nm in the deconvoluted PL spectra, respectively. Notably, a broad and intense PL emission mainly due to HIXs in the heterostructure was observed in the visible-light region at 675–700 nm (below 200 K). All of the PL peaks corresponding to the characteristic excitons in the heterostructure were red-shifted with increasing temperature owing to the enhancement of electron–phonon interactions. Interestingly, with increasing excitation power, the PL peaks of HIXs in the heterostructure were significantly blue-shifted, while those corresponding to the X0, X–, and XX of WS2 were red-shifted. These changes originate from the screening effect of the Coulomb and repulsive interactions between the dipole-aligned HIXs. Notably, the 4.22 ns lifetime of HIXs in the heterostructure obtained from the time-resolved PL decay curves at 3 K was significantly long and 8.6 times longer than that of the intralayer excitons. This study provides an understanding of HIXs in 2D heterostructures, which provide promising platforms for applications in nanoscale light-emitting diodes, sensors, and photovoltaics. [ABSTRACT FROM AUTHOR]

Micro-photoluminescence was observed while increasing the excitation power in a single GaAs quantum ring (QR) at 4 K. Fine structures at the energy levels of the ground (N = 1) and excited (N = 2) state excitons exhibited a blue shift when excitation power increased. The excited state exciton had a strong polarization dependence that stemmed from the asymmetric localized state. According to temperature-dependence measurements, strong exciton–phonon interaction (48 meV) was observed from an excited exciton state in comparison with the weak exciton–phonon interaction (27 meV) from the ground exciton state, resulting from enhanced confinement in the excited exciton state. In addition, higher activation energy (by 20 meV) was observed for the confined electrons in a single GaAs QR, where the confinement effect was enhanced by the asymmetric ring structure. [ABSTRACT FROM AUTHOR]

Temperature-, excitation wavelength-, and excitation power-dependent photoluminescence (PL) spectroscopy have been utilized to investigate the orientation-modulated near band edge emission (NBE) and deep level emission (DLE) of ZnO single crystals (SCs). The near-band-edge emission of ZnO SC with <0001> orientation exhibits strong and sharp emission intensity with suppressed deep level defects (mostly caused by oxygen vacancies Vo). Furthermore, Raman analysis reveals that <0001> orientation has dominant E2 (high) and E2 (low) modes, indicating that this direction has better crystallinity. At low temperature, the neutral donor-to-bound exciton (DoX) transition dominates, regardless of the orientation, according to the temperature-dependent PL spectra. Moreover, free-exciton (FX) transition emerges at higher temperatures in all orientations. The PL intensity dependence on the excitation power has been described in terms of power-law (I~Lα). Our results demonstrate that the α for <0001>, <1120>, and <1010> is (1.148), (1.180), and (1.184) respectively. In short, the comprehensive PL analysis suggests that DoX transitions are dominant in the NBE region, whereas oxygen vacancies (Vo) are the dominant deep levels in ZnO. In addition, the <0001> orientation contains fewer Vo-related defects with intense excitonic emission in the near band edge region than other counterparts, even at high temperature (~543 K). These results indicate that <0001> growth direction is favorable for fabricating ZnO-based highly efficient optoelectronic devices. [ABSTRACT FROM AUTHOR]

Here, we demonstrate an efficient nanocomposite system for the enhancement of optical properties via a self-assembled 2D nanoarchitecture combined with insulating and conducting polymers. The excitonic properties of the ternary composites consisting of MoS2 nanosheets, poly methyl methacrylate (PMMA), and polyaniline (PANI) (MPP) were tuned by optimizing the concentration of PANI. Their absorption spectra revealed a strong excitonic coupling between the components. The mechanisms of exciton dissociation and generation of interlayer excitons were investigated by steady-state and transient photoluminescence spectroscopy. The exciton decay time is reduced by almost 95% in the ternary composite at an optimized concentration of PANI than in pure MoS2. This indicates a fast excitonic interaction between MoS2 and PANI in the presence of the insulating polymer PMMA. Furthermore, nonlinear optical studies using the z-scan technique further confirmed the exciton–exciton coupling in the ternary composites. Therefore, these ternary composites having multiple excitons and strong saturable absorption (SA) behavior provide a perspective for next-generation saturable absorber devices and other optoelectronic applications. [ABSTRACT FROM AUTHOR]

Strong quantum confinement and coulomb interactions induce tightly bound quasiparticles such as excitons and trions in an atomically thin layer of transitional metal dichalcogenides (TMDs), which play a dominant role in determining their intriguing optoelectronic properties. Thus, controlling the excitonic properties is essential for the applications of TMD-based devices. Here, we demonstrate the all-optical tuning of the local excitonic emission from a monolayer MoS2 hybridized with phase-change material Ge2Sb2Te5 (GST) thin film. By applying pulsed laser with different power on the MoS2/GST heterostructure, the peak energies of the excitonic emission of MoS2 can be tuned up to 40 meV, and the exciton/trion intensity ratio can be tuned by at least one order of magnitude. Raman spectra and transient pump-probe measurements show that the tunability originated from the laser-induced phase change of the GST thin film with charge transferring from GST to the monolayer MoS2. The dynamic tuning of the excitonic emission was all done with localized laser pulses and could be scaled readily, which pave a new way of controlling the excitonic emission in TMDs. Our findings could be potentially used as all-optical modulators or switches in future optical networks.

The formation of structures of different sizes on the surface of a single sample and the study of the mechanisms of their excitations and energy transitions is a promising area of research. ZnO/SiO2/Si heterostructures were formed by two-stage electrochemical etching of silicon wafers and synthesizing zinc oxide nanoparticles using sol-gel technology. By scanning electron and atomic force microscopies, various pores and clusters were identified. The presence of five levels of surface hierarchy provided the synthesis of light-emitting particles with different properties. Light emission mechanisms are associated with the recombination of excitons and the hyperfine structure of charged particles trapped on oxygen vacancy.

In this paper, the behavior of exciton radiative recombination in a GaN-based triangular-like ridge cavity is studied at room-temperature. The triangular-like ridge cavity is fabricated on a standard-blue-LED epitaxial wafer grown on a sapphire substrate. Through the photoluminescence (PL) and time-resolved photoluminescence (TR-PL) measurements, a clear modulation of the original spontaneous emission is found in the microcavity, a new transition channel is observed, and the effect is angle-dependent. Furthermore, by changing the tilt angle during angle-resolution photoluminescence (AR-PL), it is found that the coupling between excitons and photons in the cavity is the strongest when tilted at 10°. By simulation, the strong localization of photons in the top of the cavity can be confirmed. The PL, TR-PL, and AR-PL results showed the sign of the exciton-photon coupling in the triangular-like ridge cavity. [ABSTRACT FROM AUTHOR]

Electronic band structure in germanium nitride having spinel structure, γ-Ge3N4, was examined using two spectroscopic techniques, cathodoluminescence and synchrotron-based photoluminescence. The sample purity was confirmed by x-ray diffraction and Raman analyses. The spectroscopic measurements provided first experimental evidence of a large free exciton binding energy De≈0.30 eV and direct interband transitions in this material. The band gap energy Eg = 3.65 ± 0.05 eV measured with a higher precision was in agreement with that previously obtained via XES/XANES method. The screened hybrid functional Heyd–Scuseria–Ernzerhof (HSE06) calculations of the electronic structure supported the experimental results. Based on the experimental data and theoretical calculations, the limiting efficiency of the excitation conversion to light was estimated and compared with that of w-GaN, which is the basic material of commercial light emitting diodes. The high conversion efficiency, very high hardness and rigidity combined with a thermal stability in air up to ~ 700 °C reveal the potential of γ-Ge3N4 for robust and efficient photonic emitters. [ABSTRACT FROM AUTHOR]

Monolayer transition metal dichalcogenides have emerged as promising materials for optoelectronic and nanophotonic devices. However, the low photoluminescence (PL) quantum yield (QY) hinders their various potential applications. Here we engineer and enhance the PL intensity of monolayer WS2 by femtosecond laser irradiation. More than two orders of magnitude enhancement of PL intensity as compared to the as-prepared sample is determined. Furthermore, the engineering time is shortened by three orders of magnitude as compared to the improvement of PL intensity by continuous-wave laser irradiation. Based on the evolution of PL spectra, we attribute the giant PL enhancement to the conversion from trion emission to exciton, as well as the improvement of the QY when exciton and trion are localized to the new-formed defects. We have created microstructures on the monolayer WS2 based on the enhancement of PL intensity, where the engineered structures can be stably stored for more than three years. This flexible approach with the feature of excellent long-term storage stability is promising for applications in information storage, display technology, and optoelectronic devices. [ABSTRACT FROM AUTHOR]

Strong quantum confinement and coulomb interactions induce tightly bound quasiparticles such as excitons and trions in an atomically thin layer of transitional metal dichalcogenides (TMDs), which play a dominant role in determining their intriguing optoelectronic properties. Thus, controlling the excitonic properties is essential for the applications of TMD-based devices. Here, we demonstrate the all-optical tuning of the local excitonic emission from a monolayer MoS2 hybridized with phase-change material Ge2Sb2Te5 (GST) thin film. By applying pulsed laser with different power on the MoS2/GST heterostructure, the peak energies of the excitonic emission of MoS2 can be tuned up to 40 meV, and the exciton/trion intensity ratio can be tuned by at least one order of magnitude. Raman spectra and transient pump-probe measurements show that the tunability originated from the laser-induced phase change of the GST thin film with charge transferring from GST to the monolayer MoS2. The dynamic tuning of the excitonic emission was all done with localized laser pulses and could be scaled readily, which pave a new way of controlling the excitonic emission in TMDs. Our findings could be potentially used as all-optical modulators or switches in future optical networks. [ABSTRACT FROM AUTHOR]

The photoluminescence and scintillation properties of n-bromophenethylammonium lead chloride ((n-BrPEA) 2 PbCl 4) (n = 4, 3, and 2) and phenethylammonium lead chloride ((PEA) 2 PbCl 4) have been investigated. All the samples showed broad emission peaks at 400–700 nm upon excitation by UV light or X-ray. The decay time constants of 8–28 ns were obtained which originated from the recombination of self-trapped exciton (STE) within inorganic layers. The afterglow levels at 20 ms after 2 ms of X-ray irradiation of (4-BrPEA) 2 PbCl 4 and (3-BrPEA) 2 PbCl 4 (76 and 55 ppm) were better than (PEA) 2 PbCl 4 (379 ppm). The light yield of (4-BrPEA) 2 PbCl 4 (3100 photons/5.5 MeV-α) was higher than (PEA) 2 PbCl 4 (1400 photons/5.5 MeV-α). • (BrC 6 H 4 C 2 H 4 NH 3) 2 PbCl 4 crystals were grown by the slow cooling method. • All the crystals showed the luminescence due to the recombination of the self-trapped excitons confined in inorganic layers. • The (4-BrC 6 H 4 C 2 H 4 NH 3) 2 PbCl 4 crystal showed the highest QY in the present crystals. • The light yields of the (4-BrC 6 H 4 C 2 H 4 NH 3) 2 PbCl 4 crystals surpassed that of the unsubstituted crystal. [ABSTRACT FROM AUTHOR]

Semiconductor single crystal ZnO nanowires have been successfully synthesized by a simple method based on thermal evaporation of ZnO powders mixed with graphite. Metallic catalysts, carrying gases, and vacuum conditions are not necessary. The x-ray diffraction (XRD) analysis shows that the ZnO nanowires are highly crystallized and have a typical wurtzite hexagonal structure with lattice constants a = 0.3246 nm and c = 0.5203 nm. The scanning electron microscopy (SEM) images of nanowires indicate that diameters of the ZnO nanowires normally range from 100 to 300 nm and their lengths are several tens of micrometers. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the nanowires were measured in the range of temperature from 15 K to the room temperature. Photoluminescence spectra at low temperatures exhibit a group of ultraviolet (UV) narrow peaks in the region 368 nm ~ 390 nm, and a blue-green very broad peak at 500 nm. Origin of the emission lines in PL spectra and the lines in PLE spectra is discussed.

The photoluminescence spectra of the InP/ZnS quantum dots (QDs) and the composite structure of the InP/ZnS QDs coupled with Au nanorods are investigated in this paper. In the luminescence spectra of the two samples, the recombination of the bright–dark doublet exciton states and an upper bright exciton state are observed at the temperature 11–130 K. The temperature dependence of the main emission peaks of the InP/ZnS QDs and the composite structure can be expressed by the Varshni expression in the temperature range of 11–300 K. The coupling between the QDs and the surface plasmon resonance (SPR) states has a great effect on the exciton recombination and the defect recombination. Because of the coupling, the variation range of the main emission peak of the composite structure is about 28 meV more than the pure QDs with the temperature from 11 K to 300 K. The red shift of the defect emission with increasing temperature is coincident with the red shift of the SPR energy with increasing temperature. The experimental results confirm that the energy transfer from the SPR states is the main source of the defect emission. [ABSTRACT FROM AUTHOR]

The protected electron states at the boundaries or on the surfaces of topological insulators (TIs) have been the subject of intense theoretical and experimental investigations. Such states are enforced by very strong spin-orbit interaction in solids composed of heavy elements. Here, we study the composite particles--chiral excitons--formed by the Coulomb attraction between electrons and holes residing on the surface of an archetypical 3D TI, Bi2Se3. Photoluminescence (PL) emission arising due to recombination of excitons in conventional semiconductors is usually unpolarized because of scattering by phonons and other degrees of freedom during exciton thermalization. On the contrary, we observe almost perfectly polarization-preserving PL emission from chiral excitons. We demonstrate that the chiral excitons can be optically oriented with circularly polarized light in a broad range of excitation energies, even when the latter deviate fromthe (apparent) optical band gap by hundreds of millielectronvolts, and that the orientation remains preserved even at room temperature. Based on the dependences of the PL spectra on the energy and polarization of incident photons, we propose that chiral excitons are made from massive holes and massless (Dirac) electrons, both with chiral spin textures enforced by strong spin-orbit coupling. A theoretical model based on this proposal describes quantitatively the experimental observations. The optical orientation of composite particles, the chiral excitons, emerges as a general result of strong spin-orbit coupling in a 2D electron system. Our findings can potentially expand applications of TIs in photonics and optoelectronics. [ABSTRACT FROM AUTHOR]

The protected electron states at the boundaries or on the surfaces of topological insulators (TIs) have been the subject of intense theoretical and experimental investigations. Such states are enforced by very strong spin-orbit interaction in solids composed of heavy elements. Here, we study the composite particles--chiral excitons--formed by the Coulomb attraction between electrons and holes residing on the surface of an archetypical 3D TI, Bi2Se3. Photoluminescence (PL) emission arising due to recombination of excitons in conventional semiconductors is usually unpolarized because of scattering by phonons and other degrees of freedom during exciton thermalization. On the contrary, we observe almost perfectly polarization-preserving PL emission from chiral excitons. We demonstrate that the chiral excitons can be optically oriented with circularly polarized light in a broad range of excitation energies, even when the latter deviate fromthe (apparent) optical band gap by hundreds of millielectronvolts, and that the orientation remains preserved even at room temperature. Based on the dependences of the PL spectra on the energy and polarization of incident photons, we propose that chiral excitons are made from massive holes and massless (Dirac) electrons, both with chiral spin textures enforced by strong spin-orbit coupling. A theoretical model based on this proposal describes quantitatively the experimental observations. The optical orientation of composite particles, the chiral excitons, emerges as a general result of strong spin-orbit coupling in a 2D electron system. Our findings can potentially expand applications of TIs in photonics and optoelectronics. [ABSTRACT FROM AUTHOR]

Abstract The biexciton, negative and positive trion states in strongly oblate ellipsoidal quantum dot are investigated in the framework of variational method and Heisenberg's uncertainty principle. Because of the oblate geometry of the quantum dot, the exciton complexes are considered as quasi-two-dimensional. The trial wave function for the biexciton and trions are constructed on the base of one-particle wave functions, which are obtained within the framework of adiabatic approximation. The dependencies of the energies and binding energies for the biexciton, negative and positive trions on the geometrical parameters of ellipsoidal quantum dot and ratio of the effective masses of the electron and hole are obtained. The recombination energies for the biexciton, negative and positive trions are considered as a function of the strongly oblate ellipsoidal quantum dot's small semiaxis. For the crossover states, it is shown that for the decreasing small semiaxis the photoluminescence emission peaks of exciton and biexciton shift to the higher energies, but the intensities remain constant. Finally, with the increase of temperature, the photoluminescence emission peaks and intensities become lower. The radiative lifetime of biexciton in ellipsoid is estimated. Graphical abstract Image 1 Highlights • The dependence of binding energies of exciton complexes on semiaxes are calculated. • The recombination energies of exciton complexes are revealed. • The lifetime of the biexciton in ellipsoidal quantum dot is estimated. • The dependence of the PL spectra of exciton and biexciton is obtained. [ABSTRACT FROM AUTHOR]

Abstract Perovskite CH 3 NH 3 PbI 3 (MAPbI 3) single crystal was grown using inverse temperature crystallization method. Crystallinity of the perovskite was confirmed by X-ray diffraction. Photoluminescence (PL) spectra revealed abnormal behavior due to a temperature-induced orthorhombic to the tetragonal phase transition. Four PL emission peaks, A, B, C, and D, were observed in the low temperature regime. Peaks A and B were observed at 756 and 776 nm at 12 K, and were blue-shifted and disappeared at 130 and 70 K, respectively. Peaks C and D were observed at 789 and 807 nm at 40 K and were also blue-shifted to 780 and 794 nm at 100 K. On the other hand, the peak C red-shifted to 799 nm from 100 to 140 K because of an orthorhombic to the tetragonal phase change and was also blue-shifted above 140 K. From the excitation intensity- and temperature-dependent PL results, peaks A and B were assigned to the free-exciton and bound-exciton of the orthorhombic phase crystal, respectively. In addition, peaks C and D were associated with the free-exciton and bound-exciton of the tetragonal phase crystal, respectively. The activation energy of peak C was calculated to be 98 meV from temperature dependence of the PL intensity. [ABSTRACT FROM AUTHOR]

Optical absorption spectra of colloidal solutions and films based on ZnSe quantum dots (QD) stabilized with thioglycerol or thioglycolic acid and precipitated on spherical nanoparticles of silicon dioxide are presented and studied. In addition to the usual quantum-size effect, two contributions to the energy of excitons in QD were found. The first (surface effect) occurs in colloidal solutions of QD stabilized by thioglycerol due to the absorption of the thiol –SH group by Zn and Se atoms. The second (polarization effect) is additional and is formed in the solid films of both colloids due to the dielectric mismatch between QD materials and the surrounding matrix. It was shown for the first time that the electronic excitation energy transfer on a spherical surface in dense ZnSe QD array occurs due to the mechanism of strong interaction (hybridization of orbitals) between neighboring QD, which is satisfactorily described by the proposed theoretical model. [ABSTRACT FROM AUTHOR]

Topologically disordered films based on binary mixtures of SiO2 nanoparticles with CdS quantum dots on their surface with different surface and volume concentrations are studied. For the first time, a percolation transition of excitons has been observed in the CdS quantum dots array on a quasi-two-dimensional surface and the features of its formation have been described qualitatively. A percolation transition of excitons in a fi lm based on a binary mixture of pure SiO2 spheres and those coated with quantum dots (SiO2/CdS), which is analogous to the phase transition of metal and dielectric macroparticles, has also been observed. It is shown that the percolation transition in such a system occurs at a critical concentration of nanoparticles of one sort (nc ~ 0.6), which is almost twice as high as in the macroparticle system (~0.29). This is due to the interaction between nanoparticles (van der Waals forces and electrostatic forces) and the chemical nature of the binding ligand. [ABSTRACT FROM AUTHOR]

Abstract We investigated the optical behaviour of excitons in monolayer MoS 2 prepared on a 60 nm-thick metal (Au) ring structure patterned on SiO 2. CVD-grown monolayer MoS 2 was transferred to the metal structure and placed in contact with three different environments simultaneously: SiO 2 , Au metal and air. The optical properties of the transferred MoS 2 on the substrates were characterized through micro-photoluminescence (μ-PL) measurements. PL peaks were observed near ∼1.82 eV for defect-related emission, ∼1.92 eV for an A exciton, which includes a neutral exciton and a trion, and ∼2.1 eV for a B exciton. The optical band gap and exciton-phonon interaction of the samples on the three substrates were investigated by measuring the temperature-dependent photoluminescence. Highlights • A chemical vapour deposition (CVD)-grown MoS 2 monolayer is transferred to the Au-patterned structure. • The monolayer MoS 2 is placed in contact with three different environments simultaneously: SiO 2 , Au, and air (free standing). • The excitonic emissions from the different contact regions were characterized by temperature-dependent μ-PL. [ABSTRACT FROM AUTHOR]

Abstract We report the synthesis and luminescence properties of new layered mixed-anion compounds, Sr 2 ScCuSeO 3 and Sr 3 Sc 2 Cu 2 Se 2 O 5. Sr 2 ScCuSeO 3 and Sr 3 Sc 2 Cu 2 Se 2 O 5 were synthesized by solid-state reaction and were investigated using x-ray diffraction, diffuse reflectance, and photoluminescence spectroscopies. Both compounds have very similar properties except for their optical band gaps and emission peak energies. At room temperature, Sr 2 ScCuSeO 3 and Sr 3 Sc 2 Cu 2 Se 2 O 5 exhibit optical band gaps energies of 2.9–3.0 eV. The excitonic luminescence with very fast emissions of ∼30 ps were observed in both compounds even at room temperature. This feature of the compounds makes them suitable for fast scintillator applications. Our results suggest that the luminescence properties such as emission peak energies and lifetimes of these layered mixed-anion compounds can be controlled by changing their structure and composition. Highlights • New layered mixed-anion compounds were successfully synthesized by solid-state reaction. • Sr 2 ScCuSeO 3 and Sr 3 Sc 2 Cu 2 Se 2 O 5 exhibit band gap energies of 2.9–3.0 eV, respectively. • Under UV excitation, Sr 2 ScCuSeO 3 and Sr 3 Sc 2 Cu 2 Se 2 O 5 exhibit a fast decay times around 30 ps even at room temperature. • Except for the emission decay, the observed optical properties are almost similar to other chalcogenide oxides. • The luminescence properties of these materials can be controlled by their composition and structure. [ABSTRACT FROM AUTHOR]

The origin of the variation of photoluminescence (PL) spectra of monolayer tungsten disulfide (WS 2 ) is investigated systematically. Dependence of the PL spectrum on the excitation power show that the relatively sharp component corresponds to excitons whereas the broader component at slightly lower energy corresponds to negatively charged trions. PL imaging and second harmonic generation measurements show that the trion signals are suppressed more than the exciton signals near the edges, thereby relatively enhancing the excitonic feature in the PL spectrum and that such relative enhancement of the exciton signals is more pronounced near approximately armchair edges. This effect is interpreted in terms of depletion of free electrons near the edges caused by structural defects and adsorption of electron acceptors such as oxygen atoms. [ABSTRACT FROM AUTHOR]